This application claims the priority of Japanese Patent Application No. 2000-180359 filed on Jun. 15, 2000, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a retro focus type wide-angle lens; and, in particular, to a small-size, large-aperture retro focus type wide-angle lens favorably employed in a CCTV (closed-circuit television) camera equipped with a CCD for monitoring, factory automation, and the like.
2. Description of the Prior Art
Since lenses employed in monitoring cameras and the like are mostly used indoors, bright large-aperture lenses are required therefor. Further, a wide-angle lens system is required for a single camera to observe or photograph a wider area.
Known as conventional examples in which such a wide-angle lens is constituted by a retro focus type in which a front group having a negative refracting power and a rear group having a positive refracting power are successively disposed from the object side are lenses disclosed in Japanese Unexamined Patent Publication No. 59-155817 and No. 9-211321. Each of these lenses is a wide-angle lens having a compact configuration and exhibiting a certain degree of performance.
However, the former lens is dark with an F number of 1.8, whereas the latter lens is dark with an F number of 2.8 and has a narrow angle of view. Monitoring cameras are required to be used in darker places, whereby the advent of brighter lenses is waited for.
In view of such circumstances, it is an object of the present invention to provide a retro focus type wide-angle lens which is brighter than the conventional examples while being compact and maintaining favorable optical performances, and a lens apparatus using the same.
The present invention provides a retro focus type wide-angle lens comprising, successively from an object side, a front group having a negative refracting power and a rear group having a positive refracting power; the front group comprising, successively from the object side, a first lens made of a negative meniscus lens having a convex surface directed onto the object side, a second lens made of a biconvex lens, a third lens made of a negative meniscus lens having a convex surface directed onto the object side, and a fourth lens made of a positive meniscus lens having a concave surface directed onto the object side; the rear group comprising, successively from the object side, a cemented lens in which a fifth lens made of a negative lens and a sixth lens made of a positive lens are cemented together, and a seventh lens made of a positive lens having a convex surface directed onto the object side; the retro focus type wide-angle lens satisfying the following conditional expressions (1) and (2):
0.6 less than ƒxe2x80x2/I less than 0.85xe2x80x83xe2x80x83(1)
0.45 less than |ƒxe2x80x2G1/DG1-2| less than 0.88xe2x80x83xe2x80x83(2)
where
fxe2x80x2 is the focal length of the whole system;
I is the diagonal length of an effective screen;
fxe2x80x2G1 is the focal length of the system formed by the first to third lenses; and
DG1-2 is the distance from the object-side surface of the first lens to the imaging-side surface of the fourth lens.
Preferably, the retro focus type wide-angle lens of the present invention satisfies the following conditional expression (3):
0.2xe2x89xa6DG2/DG1-2xe2x89xa60.5xe2x80x83xe2x80x83(3)
where
DG1-2 is the distance from the object-side surface of the first lens to the imaging-side surface of the fourth lens; and
DG2 is the center thickness of the fourth lens.
A stop for restricting a marginal luminous flux may be disposed between the third and fourth lenses, whereas an aperture stop may be disposed between the fourth and fifth lenses.
Preferably, the retro focus type wide-angle lens has an F number set within the range from 1.25 to 1.43.
The present invention also provides a lens apparatus comprising the retro focus type wide-angle lens in accordance with the present invention.